1) See as many examples as possible of good personal statement to get an idea of the structure and things to include
2) Bullet point it first. It makes it so much easier to write out a bullet point list of all your achievements and reasons why you want to do the course. It also makes sure you don't miss anything out later on.
3) Key things to include: Why you want to do that course and Why you are suited to do that course.
4) Do many drafts. Don't worry too much if it's not great at first, or over the word limit. You'll write and re-write it, cut it down and include more.
5) Remember you might be asked to talk about things you have mentioned in your personal statement, should you be invited to interview. So everything you include, you need to be comfortable talking about.
6) Research the Course using its website. It will tell you the areas it studies and you can use this information to tailor your interests to it. For example, if I was applying to do a History degree at the Mentor Univeristy, and the vast majority of the course was about medieval history, it would seem a bit silly to only say in your personal statement how much you were interested in WWII.
7) Make it relevant to all courses you apply for. This balances out the point above a bit, because although you want to tailor it to a particular choice, your first choice, 5 different Universities see it and they don't know your other choices.
8) Go to University Open Day's, they usually give talks on how to do Personal Statements and what in particular they want to see in the ones they get.
9) If you can link the course you are applying for to your future career plans, it looks like your choice is a well thought out decision. If you can't link them, maybe you should wonder why you picked it?
10) Get as many people to read it and check over it as you can before you submit it. Hopefully this will weed out any spelling mistakes or anything that just doesn't quite sound right.